Semiconductor materials, in the form of wafers, are used extensively as substrates in electronic applications. Typically, semiconductor wafers are processed using conventional techniques to form semiconductor devices. Ion implantation is one conventional technique for introducing dopants into semiconductor wafers. Dopants may be implanted into a semiconductor to form regions of desired conductivity. Such implanted regions can form active regions in the resulting semiconductor devices.
Typically, during ion implantation, a dopant gas is ionized in an ion source to form positive ions and electrons. The positive ions are accelerated at a selected energy to form an ion beam. The beam is directed at the surface of the wafer and the impinging ions penetrate into the bulk semiconductor material to form an implanted region having a desired conductivity.
In certain implantation processes, it is desirable to use an ion beam having a neutral space charge, that is, a beam in which the charge contribution from positive ions is substantially equally to the charge contribution from electrons. Beams that do not have a substantially neutral space charge (i.e., beams that have either a positive or a negative space charge) may have a tendency to diverge, or expand. This divergence makes it difficult to transport such beams through the system to the wafer. Consequently, the beam current delivered to the wafer may be reduced, thereby increasing implant time. Furthermore, the uniformity of dopant implantation over the implant region can be limited. Also, non-neutral beams may lead to charge build up on the wafer surface which can damage or destroy the wafer.
In particular, generating an ion beam at low energies (e.g., less than 5 kV) having a neutral space charge may be challenging. Low energy beams are used, for example, in ion implantation processes that form doped regions having shallow depths. Typically, low energy beams have more positive ions than electrons, thus giving the beam a net positive charge. Also, low energy beams travel more slowly and, thus, have more time to diverge.
One conventional technique for neutralizing an ion beam involves increasing the flow of the dopant gas to be ionized within the ion source. However, such dopant gases (e.g., BF3) are relatively expensive and, thus, such techniques can significantly increase the cost of the process. Also, increasing the dopant gas flow rate may require high arc currents within the source which can reduce the source lifetime. In some cases, increasing the dopant gas flow rate also leads to arcing within the extraction gap which can cause process instabilities.
Other known techniques involve the use of electron flood guns or plasma flood guns which introduce electrons into the beam to neutralize charge buildup on the semiconductor wafer. Typically, electrons are introduced into the beam in a region proximate the wafer to optimize charge neutralization on the wafer.